A number of electronic components depend upon accurate reference currents to provide reliable results. Precision current references are often utilized in a variety of applications including precision delay stages, current sensing circuits for read paths in memories and timing circuits. However, maintaining relatively high accuracy and precision of such reference currents across process voltage and temperature (PVT) variations is usually very difficult and expensive.
Non-volatile memory macros with minimum area for embedded applications often utilize a reference current. These applications often use single ended memory bit cells instead of differential bit cells to conserve area. Relatively precise current references are utilized in the read sense path in order for such memory macros using single ended sensing schemes to have a fast read access along with good sense margin. These relatively precise current references are then used to design precision delay stages for use in the sense timing path or as a reference current to compare against in a current sensing scheme.
Traditional attempts at providing a precise current reference are usually expensive and often limited in precision. For example, given a zero temperature co-efficient (TC) voltage reference (e.g., a band gap reference), the classical way of getting a current reference is by designing a circuit that will give a current of Vref/R. FIG. 1A shows a conventional circuit for a current reference using a constant voltage reference. One way to keep the reference current constant with temperature variation is to use a resistor with zero temperature co-efficient and a reference voltage with zero temperature co-efficient.
Unfortunately, resistors with zero temperature coefficients are in general external resistors and expensive to implement. On the other hand if on-chip resistors are used, the current reference becomes affected by the temperature coefficients of the on-chip resistors. As a result, accuracy of on-chip resistors is very low, e.g., between a % and ±15%. Moreover, the total accuracy across all PVT corners is between ±20% and ±25%.
In other traditional attempts, a current reference based on a proportional to absolute temperature (PTAT) voltage applied to a positive TC resistor (e.g., serial and parallel combination of resistors) is utilized. The resulting resistor should have the TC equal to the PTAT voltage (TCR=TCPTAT). FIG. 1B shows a conventional circuit for a current reference based on an equivalent resistor. The current reference is constant if the resistor's TC equals the PTAT voltage TC as provided by the ΔVbe of the two bipolar junctions. Unfortunately, this condition is difficult to satisfy in practice because TC of ΔVbe has only linear terms proportional to T whereas the TC of the resistor has an offset, linear and higher order polynomials terms. Thus, this circuit requires expensive external resistors or a very critical process controlled on chip resistor. Moreover, processes in general do not use resistors with large TC as the PTAT voltage.
Another conventional attempt utilizes an Oguey's Current Reference (Ref: “CMOS Current Reference Without Resistance” by Henri J. Oguey, vol. 32, No. 7, p. 573, July 1997). There are other current references based on Vt (e.g., threshold of MOSFET) where in the final current reference is given by Vt/R. The current references mentioned above usually require either special off-chip expensive resistors or vary a lot across PVT. FIG. 1C shows a conventional Oguey's current reference circuit. Unfortunately, the Oguey's circuit has approximately T0.5 dependence on temperature presuming that the mobility varies as T−1.5 for the temperature ranging between −40 C to 145 C. Thus, the accuracy become ±15% across this temperature range and the total accuracy varies between ±20% and ±25% across all PVT. Unfortunately, the Oguey circuit uses transistors in sub-threshold region, which leaves the circuit vulnerable to mismatch related inaccuracies.